JPH0614522Y2 - Demodulation circuit for car telephone equipment - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention relates to a demodulation circuit for a car telephone device.

[Outline of device]

This invention is a circuit for demodulating and detecting received data and a monitoring signal during line connection in an automobile telephone, and is configured by combining a low-pass filter and a high-pass filter composed of a switched capacitor filter. In the case of car telephone systems of different formats, the clock frequency of only the low-pass filter is switched to prevent malfunction in detection of the monitoring signal.

[Conventional technology]

In a so-called cellular car telephone, data for channel setting and the like are exchanged between a base station and a mobile station when connecting lines or moving between cells. Also, during the line connection, the monitoring signal SAT (Supervisory A
Confirm each other by exchanging sine signals called udio Tone).

This SAT is a 6 kHz signal and must be detected and demodulated separately from the voice data of the conversation during the line connection. In this case, the voice data and the SAT spectrum have the characteristics shown in FIG.

FIG. 3 shows a filter for demodulating and detecting this data and SAT. The data received through the input terminal (3) is band-limited by the low pass filter (1) in the first stage and converted into an analog signal. . This received data output is taken out through the output terminal (4), and the audio data is taken out by an audio filter (low-pass filter) in the band up to 3 kHz (see the characteristic of the broken line (7) in FIG. 4).

The low-pass filter (1) is the next high-pass filter.
In combination with (2), a bandpass filter (see the characteristic of the broken line (8) in FIG. 4) is constructed, whereby SAT is detected and this SAT is derived at the output end (5).

Here, for the above filters (1) and (2), a switched capacitor filter is exclusively used for downsizing and low power consumption. (6) is an input terminal of the sampling clock supplied to these filters (1) and (2), for example, 400
A clock of kHz is used.

[Problems to be solved by the invention]

By the way, cellular car phones have different formats depending on the region, and the data transmission rate is 10kBps in the AMPS standard corresponding to the North American region, and 8kBps in the TACS standard corresponding to some regions in the UK and Japan. There is. However, both of them are set to 6 kHz for SAT.

Therefore, in the circuit of FIG. 3, it is necessary to change the cut-off frequency _c1 of the low-pass filter (1) in order to cope with this difference in the data transmission rate. For example, in the AMPS standard, _c1 = 13 kHz, in the TACS standard, _c1 = 10.4kHz. Since the SAT is the same in both standards, the cutoff frequency _c2 of the high pulse filter (2) does not have to be changed.

In the case of using the switched capacitor filter of the circuit of FIG. 3, the sampling clock frequency of the switched capacitor filter may be changed to cope with the difference in the transmission speed. However, in the circuit of FIG. 3, the clock is common to the filters (1) and (2). Therefore, when the sampling clock frequency of the switched capacitor filter is changed, the cutoff point of the high-pass filter in the SAT detection bandpass filter is changed. Shifts downward, and is affected by the voice band (up to 3 kHz), causing erroneous SAT detection (see the characteristic indicated by the broken line (9) in FIG. 5).

On the other hand, it is possible to change the coefficient of the low-pass filter (1) in the first stage between the AMPS system and the TACS system without changing the clock frequency.

That is, FIG. 6 shows an example of the configuration of the low-pass filter (1), which is an example of the fourth-order Butterworth type. SW
_{1 to} SW ₈ are switches that are switched by a sampling clock, OP _{1 to} OP ₄ are non-inverting operational amplifiers, and the capacitances of capacitors C _{1 to} C ₄ connected between the input and output ends of the operational amplifiers OP _{1 to} OP ₄ are AMPS. For system and TA
You can switch between the CS method and the one for the CS method.

However, this complicates the circuit configuration and increases the chip size.

[Means for solving problems]

The demodulation circuit of the car telephone device of the present invention is supplied with a reception input, and is composed of a low-pass filter (11) composed of a switched capacitor filter, and a high-pass filter (connected to a subsequent stage of this low-pass filter, which is composed of a switched capacitor pasta filter ( 12) and equipped with a high-pass filter (12)
The signal for monitoring SAT being connected to the line is obtained as the output of, and the received data output is obtained from the output of the low pass filter (11).
The frequency of the clock supplied to the low-pass filter (11) is not changed in the case of the mobile phone of the standard and the case of the mobile phone of the second standard, without changing the frequency of the clock supplied to the high-pass filter (12). Try to change.

[Action]

Only the clock frequency for the low-pass filter (11) is changed according to, for example, the AMPS standard and the TACS standard, and the clock frequency for the high-pass filter (12) is not changed. Therefore, even if the bandpass bandwidth for SAT is changed, The cutoff frequency does not change, and the voice band does not influence the detection of SAT.

〔Example〕

FIG. 1 shows an embodiment of the present invention, in which (11) is a low pass filter and (12) is a high pass filter, both of which are switched capacitor filters.

Further, (13) is an input end of the received data, and the received data is supplied to the low pass filter (11). Reference numeral (14) is an output end of the received data output derived from the output end of the low-pass filter (11), where an analog signal band-limited by the filter (11) is obtained.

The high-pass filter (12) constitutes a band-pass filter in combination with the low-pass filter (11), and the SAT in the received data is obtained at the output terminal (15) derived from the output terminal thereof.

And in this example, if it is AMPS standard, switch (18)
Is switched to the terminal A side, for example 400kHz as before.
The sampling clock CKA is supplied to the low-pass filter (11) and the high-pass filter (12) through the input terminal (16).

On the other hand, in the case of the TACS standard, the switch (18) is switched to the terminal T side. And at this time, the high-pass filter (12)
The sampling clock for the low pass filter (11) is not changed by the clock CKA, but the sampling clock for the low pass filter (11) is changed to the clock CKT through the input terminal (17) which changes the cutoff frequency _c1 so as to meet the TACS standard.

Therefore, when the standards are different, the cutoff frequency _c1 of the low-pass filter (11) can be changed so as to match the received data of the standard, and good reception data output can be obtained.

And even when the standards are different, the high-pass filter (12)
Since the cut-off frequency _c2 does not change, the signal in the voice band does not enter the band pass band formed by the filters (11) and (12), and the SAT detection does not malfunction.

FIG. 2 shows another embodiment of the present invention, in which the filter (1
1) and the filter (12) are supplied with separate sampling clocks.
According to the standard, the clock CK ₁ is supplied, while the filter (1
Clocks CK ₂ and CK ₃ are switched and supplied to 1) by a switch 19 which is switched according to the standard.

It can be easily understood that the same effects as those of the example of FIG. 1 can be obtained in this example as well.

[Effect of device]

In this invention, of the low-pass filter and the high-pass filter composed of the switched capacitor filter, only the sampling clock of the low-pass filter is
As the AMPS and TACS standards are changed, the cutoff frequency on the low frequency side of the bandpass filter characteristics for SAT detection does not change between the two standards, and when the cutoff frequency of the lowpass filter is changed according to the standards. Also, no malfunction occurs in the detection of SAT.

[Brief description of drawings]

FIG. 1 is a circuit diagram of an embodiment of the present invention, FIG. 2 is a circuit diagram of another example of the present invention, FIG. 3 is a circuit diagram showing a configuration of a conventional demodulation circuit, FIGS. Is a diagram for explaining the frequency spectrum and filter characteristics of received data,
The figure is a circuit diagram of an example of a switched capacitor filter. (11) is a low-pass filter, (12) is a high-pass filter, (1
3) is an input end of received data, (14) is an output end of received data output, (15) is an output end of SAT which is a monitoring signal, (16) and
(17) is an input terminal of the sampling clock.

Claims (1)

[Scope of utility model registration request] 1. A low-pass filter, which is supplied with a reception input and is composed of a switched capacitor filter, and a high-pass filter, which is connected to a subsequent stage of the low-pass filter and is composed of a switched-capacitor filter, as an output of the high-pass filter. The signal for monitoring during line connection is obtained, the received data output is obtained from the output of the low-pass filter, and the signal is supplied to the high-pass filter in the case of the first standard mobile phone and the second standard mobile phone. A demodulation circuit for a car telephone device, wherein the frequency of the clock supplied to the low-pass filter is changed without changing the frequency of the generated clock.